1. Field of the Invention
The present invention relates generally to allocation of wireless resources in a distributed antenna system using a dedicated line and bandwidth. More particularly, the present invention relates to a scheduling apparatus and method for effectively applying technologies related to cooperative transmission between antennas, which is based on prior cooperative information, to channels individually to improve transmission efficiency of a forward link in a multi-channel distributed antenna system.
2. Description of the Related Art
The continuous development of wireless communication systems requires technological advancement in efficient operation and distribution of frequency resources. Related research is presently being conducted in the field of a multi-hop transmission scheme, extended from the existing single-hop transmission scheme, which allows only direct transmission from one Base Station (BS) per cell to a Mobile Station (MS). In a system supporting the multi-hop transmission scheme, a signal from a BS is generally transmitted to an MS via a Relay Station (RS), and direct transmission from the BS to the MS is also possible.
Examples of cellular systems may include a single-hop system, a wireless single-hop system supporting repeaters, and a wireless multi-hop system supporting RSs. In the single-hop system, there is one BS per cell without any repeater, and a terminal, or an MS, is directly connected to the BS without a separate relay operation. The wireless single-hop system supporting repeaters is realized through the additional installation of repeaters between the single-hop systems to improve signal reception performance of MSs located in a cell boundary region or a shadow region. In this system, one cell includes one BS and several repeaters, and an MS can simultaneously transmit and receive signals to/from the BS and the repeaters.
In the wireless single-hop system supporting repeaters, a repeater amplifies not only the signal from a BS but also amplifies interference input from an external cell and transfers the amplified signal. In the wireless multi-hop system supporting RSs, an RS not only can amplify the signal desired by an MS, but also can perform scheduling and dynamic channel allocation on MSs in a sub-cell formed by the RS. Through the use of the wireless RS, the BS can transmit data even to the MSs located in the shadow region where it can hardily transmit the data, thus contributing to an extension of cell coverage and an increase in cell throughput.
As described above, the RS-based multi-hop system may have improved transmission performance compared with the single-hop system or the wireless single-hop system supporting repeaters.
In addition, the RS-based multi-hop system can be regarded as a distributed antenna system that takes charge of one cell together with a BS and RSs. In the distributed antenna system, the BS and the RSs serve as antennas that transmit signals.
In this regard, handoff can be considered as a cooperative transmission scheme between antennas in the current cellular system. The handoff is provided to allow a particular MS to move from a communication area of a particular BS or RS to a communication area of an adjacent cell's BS, without call interruption, by switching to a channel of another BS, i.e. the adjacent cell's BS. The handoff can be roughly classified into two types: hard handoff and soft handoff.
In the hard handoff, if signal strength of the current cell is less than or equal to a predetermined threshold while a particular MS moves to a region of an adjacent cell, the MS cuts off its connection to the old cell and is connected to a channel of a new cell.
In the soft handoff, if signal strength of a new cell is greater than or equal to a predetermined threshold while a particular MS travels to a region of an adjacent cell, the MS simultaneously receives signals transmitted from two BSs. When signal strength of the current cell is less than or equal to a predetermined level, the MS cuts off its connection to the current cell, and receives signals only from the new cell.
The hard handoff may experience call interruption in an environment where a particular MS leaves a service area of the current cell and travels to another cell, but the use of the soft handoff can prevent the call drop by handing over the particular MS to a service area of the new cell. Though the soft handoff prevents the communication failure caused by the MS's movement, it cannot guarantee improvement of transmission capacity of the system in view of the whole cell.
Advanced inter-antenna cooperative transmission technologies, such as Signal Combining and Space-Time Coding, can increase cell transmission efficiency and may be effective in decreasing outage probability due to degradation of signal quality in the cell boundary region. When a particular MS receives a service from two or more RSs, its signal quality and transfer rate increases, compared with when the MS receives a service from one RS. When Single Transmission (ST) is performed in the cell boundary region, the signal quality decreases as an interference signal from an adjacent cell is high in power, probably disabling communication of a certain MS. When cooperative transmission is performed by RSs, the interference source decreases in number and the strength of a received signal increases, enabling the communication and thus contributing to a reduction in an outage probability of the entire cell. However, when N RSs perform cooperative transmission, they may undesirably consume the wireless resources used for single transmission, N times more than usual.